A hydraulic tensioner applies tension to a timing chain by a plunger that slides in, and protrudes from, a plunger-receiving hole in a tensioner housing, and that, with the housing, forms a high-pressure oil chamber. Oil is introduced under pressure into the high-pressure oil chamber from outside the housing, and the plunger is urged in the protruding direction both by a spring and by hydraulic pressure. An example of a hydraulic tensioner is described in U.S. Pat. No. 5,879,255, granted Mar. 9, 1999.
The tensioner comprises a housing, a cylindrical chamber formed within the housing, a plunger movable axially in the cylindrical chamber, a spring urging the plunger in a protruding direction, a high-pressure oil chamber formed behind the plunger, a oil passage communicating with the high-pressure oil chamber, and a check valve for closing the oil passage when the pressure within the high-pressure oil chamber becomes higher than the oil supply pressure. An air bleeding orifice is provided in the housing for communication between the high-pressure oil chamber and the exterior of the housing. The air bleeding orifice can be formed by a threaded hole in the housing 1, and a screw threaded into the threaded hole. In such a case, both flanks of the screw threads engage flanks of the threads of the threaded hole, and the air bleeding orifice is composed of a microscopic helical clearance formed between the ridges of the threads of the threaded hole and the bottoms of the mating threads of the screw.
When air is present in the high-pressure oil chamber of a hydraulic chain tensioner at the time the engine is started, the compressibility of the air allows the timing chain to generate flapping noises. The prior art chain tensioner described above is unable to bleed air from high-pressure oil chamber reliably through the microscopic helical thread clearance, and is therefore unable to eliminate the generation of flapping noises.
The washer interposed between the head of the screw and a surface of the housing is a split lockwasher having a radial gap that serves as an air passage. However, it is difficult to set the gap when inserting the screw. Therefore the tensioner is unable to regulate the bleeding of air from the high-pressure oil chamber. In addition, because of the presence of the gap, the spring may not exert sufficient force to lock the screw adequately.